If Switch really sells 15M this fiscal year ending March '24, then it should sell about 10M in the FY ending March '25. I don't think that Nintendo accept sales below 10M.
I still think holiday 2024, but March 2025 is possible as well. Holiday 2025 would be a bit late I guess, but it's not completely out of the question.
At this point I think Nintendo already has a new open world 3D Mario and a new Mario Kart 90 % finished and they are waiting to release them on the new console. No way that we'll see those games on the current system.